external cython packages broken in 10.8.beta1

[dimpase]

As reported by @mezzarobba on sage-devel
The problem appears in the current built, as well as in the Meson build (#39030).
(I didn't try non-editable builds though)

The problem is that the Cython headers are not properly provided to the builder.
In the case reported, with https://github.com/abelfunctions/abelfunctions, it suffices to create a link to the sage source tree in
the root directory of the package, i.e.

git clone  https://github.com/abelfunctions/abelfunctions
cd abelfunction
ln -s <full path so the sage install>/src/sage .
sage --pip install .

[tobiasdiez]

I think there is no good way to find the path to the headers in an editable install:

meson-python: scipy/scipy#20540 (comment)

Editable installs are fundamentally incomplete - they are only guaranteed to work at the Python level (import scipy etc. works), but things like generated headers (which is what .pxd files are) cannot be installed in the right place.

setuptools: https://setuptools.pypa.io/en/latest/userguide/development_mode.html

There is no guarantee that attributes like path or file will correspond to the exact location of the original files (e.g., setuptools might employ file links to perform the editable installation). Users are encouraged to use tools like importlib.resources or importlib.metadata when trying to access package files directly.

Probably the best we can do is check for an editable install (via IS_EDITABLE = 'editable' in sage.__path__[0]) and in this case return the value of SAGE_SRC as well. This will most likely not work for auto-generated headers (those will be in the build folder), but I'm not sure if there are any/many.

[jeriedel24]

A workaround that works for me:
Set the Python search path with every invocation of make.
In my build environment I used to set: export MAKE="make -j8"
Now I set: export MAKE="make -j8 PYTHONPATH=/full/path/to/sage-10.8.beta1/src"

[user202729]

is it the same issue as sagemath/sagemath-giac#2 ? I thought it never worked.